According to the known state of the art, there are low-voltage circuit breakers of various types, depending on the number of poles (i.e. single- or multiple-pole), or depending on the type of operation (e.g. power line circuit breakers, residual current circuit breakers for detecting fault currents to earth, disconnector circuit breakers, and so on). These circuit breakers are generally characterized in that they are associated with electric or electronic devices, such as protection devices that automatically open the circuit breaker in the event of certain conditions (overloads, short circuits or anomalies), signal devices, communication devices, or general accessories such as release devices, spring-loading devices, opening and closing coils, and so on.
In order to function properly, said electric or electronic devices generally need a power source to be provided at a given voltage, with the normal allowable tolerance, that must assure the availability of a power supply PS compatible with the absorption of all the devices, or auxiliary loads (Aux. Loads) involved. Said power supply PS can be delivered by a suitable auxiliary network or batteries, or by specific power supply units capable to take power directly from the network on which the circuit breaker is installed.
Conventionally, there are various types of power supply device for taking energy directly from the network in which the circuit breakers and associated devices are installed.
In a first type, the power supply device takes power from the main power network by means of voltage transformers and/or rectifier bridges, and subsequently regulates said power supply by means of a suitable network of passive and/or discrete elements. In practice, this type of approach is very sturdy, and is capable of providing a generally adequate power supply PS, but it has certain drawbacks. The use of a network of passive and/or discrete elements makes it difficult to guarantee stable power supply conditions for an indefinite period of time, especially in the event of variations in the power supply PS demanded by the auxiliary loads and/or a variability of the phase voltage VPH.
In fact, this solution relies on the use of devices that are always governed at a specific switching frequency fSW, and that are designed to provide a voltage at a value near a preferred reference voltage (VR) with circuits operating on the basis of variations in the so-called duty cycle, i.e. by means of cyclic “on-off” sequences at the switching frequency fSW of the process for charging batteries, such as capacitors. It is common knowledge that the voltage at the terminals of a capacitor depends on its charge according to the formula:V=Q*C 
where: V is the voltage, expressed in Volts [V], for instance; Q is the charge, expressed in Coulombs [C], for instance; and C is the capacity of the capacitor, expressed in Farads [F], for instance.
Moreover, from the formulas:IC=dQ/dt=CdV/dt 
or, inversely
      V    ⁡          (      t      )        =            1      C        ·                  ∫        0        t            ⁢                        I          ⁡                      (            t            )                          ⁢                  ⅆ          t                    
we can see that every time a current ICL circulates in a capacitor CL, the charge Q and consequently the voltage at its terminals change; this happens, for instance, in the case of variations in the absorption of the devices being powered (Aux Loads) or changes in the phase voltage (VPH). The method thus consists in measuring said voltage and cyclically governing series of current pulses at a specific, fixed switching frequency fSW in the capacitor CL until the preferred voltage is restored at its terminals, basically submitting the capacitor to a process of cyclic charging at a fixed switching frequency fSW, and regulating the corresponding duty cycle. More precisely, given that a period of time TSW expressed in seconds physically equates to the inverse of the switching frequency fSW expressed in Hertz, according to the known relation TSW=1/fSW, the process for charging the capacitor can pass from a virtually nil minimum value to a maximum value and back again to a minimum value for every period TSW of the feedback oscillator (Clock & Duty cycle control). In practical terms, when the voltage is lower than the preferred value, the duty cycle acquires high values (i.e. long “on” fractions in the period TSW), whereas when the voltage is too high, the duty cycle takes on the lowest values technically allowable.
It has been demonstrated, however, that the tolerances and physical limits of the components used and consequent phenomena of inertia, hysteresis, transition and thermal drift mean that the alternate switching between “on” and “off” states does not take place at exactly the right time: it is not instantaneous, but occurs in far from negligible fractions of the period TSW. The fact that it is impossible to obtain instantaneous on-off cycles thus sets the lower limit for regulating said duty cycle, which can typically vary effectively enough only between theoretical values coming between 10% and 98%. As a result, the voltage can only be partially limited on the basis of the instantaneous correction of the duty cycle at a given switching frequency fSW, and said regulation cannot be sufficiently accurate.
Outside the narrow operating range, further specific dissipating devices prevent any uncontrolled increase in the voltage at the capacitor's terminals by taking energy from the capacitor and converting it into heat. The large number of passive and discrete components required may also become a critical issue in terms of overall dimensions and costs, however. In practice, it has been demonstrated that the proper operation of the system is protected against the substantial inefficiency of these solutions only for limited dynamics of the phase voltage VPH, roughly between −30% and +100% of the rated value, and it is consequently essential to provide for differently-dimensioned power supply devices to suit the network voltage being used. The use of power supply devices based on this type of approach is consequently scarcely flexible in relation to the network voltage, entailing a consequent increase in production, installation and operating costs. Moreover, the high energy consumption statistically observable due to dissipation induces a considerable heating and greater deterioration of the components, as well as having a far more negative fallout on the energy balance of the main network than would be due to the power supply actually delivered for the auxiliary loads. This poor energy efficiency can give rise to temperatures as high as 140° C. in some of the components involved.
Another drawback of the power supply devices of the previously-described type concerns the so-called startup time TSU, i.e. the time it takes to reach a steady state: being relatively long, this can become a critical issue when it comes to powering a protection device, for instance. Problems can occur, for example, when the circuit breaker is closed in latent short-circuit conditions: in such cases, any delay in the power supply to the protection device tends to be translated into damage to the system and to the circuit breaker.
A second type of power supply device is structured so as to conduct power directly from the sensors used to record the value of the line currents IL. This solution is applicable, for instance, in the case of circuit breakers based on the use of current transformer (CT) sensors, which are suitable for simultaneously providing signals indicative of the value of the current circulating in the phases and powering the electric and electronic devices associated with the circuit breaker.
This second type of solution, as described in the German patent DE19819149 for instance, offers the undeniable advantage of not depending on the phase voltage VPH, but it nonetheless reveals considerable drawbacks. In fact, the power supply voltage VPS that can be delivered by the power supply device is influenced in this case by the value of the line currents IL, which is known to be the most variable parameter in a normal electric circuit. The power conducted from the network consequently cannot be transferred directly to the auxiliary loads in this case either. In much the same way as described above for the first type of device, here again in this second case, phase shifters governed at a constant switching frequency fSW are used with means for dissipating any excess energy so as to compensate for any variations in line current IL and in the power supply PS absorbed by the auxiliary loads. So the drawbacks relating to the inefficiency of the so-called duty cycles at a constant frequency remain much the same.
A further drawback of the power supply devices of the above-described type lies, here again, in the lengthy startup time TSU. In fact, since the initial value of the line currents IL is generally not known, it is impossible to have any real guarantee; at best, the minimum time always ranges from 16 ms to 20 ms and, for the reasons previously explained, that can hardly be considered satisfactory, especially for the purpose of protection devices.
Another limit intrinsic in the use of current transformer sensors lies in that they can only work in networks powered in AC, not in applications in DC.
Another drawback common to both types of solution described above derives from the fact that the optimal switching frequency fSW of the shifters is around a few thousand Hertz, and therefore considerably higher than the mains frequency (which is normally 50 or 60 Hz), and this circumstance is translated into a considerable risk of electromagnetic disturbance relating, for instance, to the signals controlled by a protection device, but also in the form of unwanted backflows into the main electric network. This phenomenon is illustrated in the oscillogram in FIG. 1.
Another disadvantage common to the two known solutions previously described lies in their low general performance or energy efficiency (which becomes even worse when the dissipators take effect); the energy consumption of the power supplies described may even be sufficient to cause a considerable disruption in the energy balance of the stretch of power network where the circuit breaker is installed with its related devices to power.